ARINC (Aeronautical Radio, Incorporated) 429 specification, also known as Mark 33 Digital Information Transfer System (DITS) specification, defines the standard requirements for the transfer of digital data between avionics systems on commercial aircraft. The specification defines the physical layer electrical characteristics, data word structure, and the timing and protocol necessary to establish a bus communication link. For instance, in accordance with the current ARINC 429 specification, the physical layer media is simplex, using a 780 shielded twisted pair cable. The link uses a Return-to-Zero (RZ) line-code or modulation. In RZ modulated transmission each bit cycle time ends with the signal level at 0V or equivalently at the NULL level. The transmitter drives the bus differentially with controlled transition slew-rates and a prescribed output source impedance such as 75Ω±5Ω.
Typically, ARINC 429 communication links are deployed in multi-voltage environments plagued by ground loops and are susceptible to transient surge voltages induced by lightning incident on the airframe. Commercial aircraft are relatively large so the ARINC 429 line drivers and line receivers often do not work in close proximity, which leads to finite impedance differences in the transmitter and receiver grounds (i.e., ground loop). This finite impedance differences along with vast and complex electrical power systems of the various ARINC 429 interconnected avionic equipment can create large common-mode bus voltage and power/ground short circuit or glitch hazards resulting in receiver damage. These power faults may propagate to the delicate and expensive mission critical subsystems that are processing the ARINC 429 bus data.
RTCA (Radio Technical Commission for Aeronautics)/DO-160G section 22 specifies the susceptibility of a device to lightning induced transient surge voltages. The effects of lightning on commercial aircraft are highly regulated worldwide today, due to the increased use of composite materials, such as Carbon Fiber Composite (CFC). In fact, CFC is now widely used for recently introduced airframe designs such as Boeing® 787 and Airbus® A380 airframes. The lightweight property of CFC is very attractive to the aerospace industry where weight/thrust ratios are critical for operation, maneuverability, and fuel efficiency.
Prior to the use of CFC materials, the airframe and most other parts of the airplane were made of metal. Thus, if a lightning strike occurred at the nose of the plane, during takeoff for instance, the lightning would travel outside the plane to the tail, exit the surface of the plane, and terminate to ground. The solid metal construction of the airframe acted as a Faraday cage, providing an extremely low impedance path for the lightning current. This prevented coupling of voltages and currents to the internal ARINC 429 bus cable in the plane, which sometime is routed along the side of the plane, between the inside of the outer skin and the interior bulkhead, and throughout the wing. This also greatly reduced the susceptibility of the mission critical components located inside the plane connected via ARINC 429. CFC materials, however, do not conduct lightning currents the way metal airframes do. As a result, the increased impedance of the outer skin as a path for the lightning increases the possibility of higher voltages and currents coupling directly onto internal ARINC 429 cables, and thus to the airborne utilization equipment they link. The result could be catastrophic.